Chronocort Versus Plenadren Replacement Therapy in Adults With Adrenal Insufficiency

Inclusion Criteria: * Male or female participants must be aged 18 years or older at the time of signing the informed consent. * Participants with known (documented) primary AI, defined as early morning pre-dose cortisol \<50 nmol/L and currently treated with glucocorticoid as replacement therapy. Primary AI includes any cause of acquired or congenital primary adrenal failure including autoimmune Addison's disease and bilateral adrenalectomy (except when performed for Cushing's syndrome). * Participants on stable glucocorticoid treatment for ≥3 months prior to the Screening Visit. * Participants on a stable dose of fludrocortisone (if applicable) for ≥3 months prior to the Screening Visit. * Male participants must agree to use contraception as detailed in Appendix 4 of the protocol, during the Screening, Treatment, and Follow-up Periods and refrain from donating sperm during these periods and for 7 days after the last dose of study treatment. * A female participant is eligible to participate if she is not pregnant, not breastfeeding, and for whom at least one of the following conditions applies: not a woman of childbearing potential (WOCBP), or a WOCBP with a negative urine pregnancy test at entry into the study who agrees to follow the contraceptive guidance during the Screening, Treatment and Follow-up Periods and for 7 days after the last dose of study treatment. Note: Females presenting with oligomenorrhea or amenorrhea who are ≤55 years should be considered potentially f…